Effect of MOSFET parameter mismatch on current distribution and power dissipation imbalance

经过
军医都市，高级应用工程师，英飞凌技术

Introduction
当几个power MOSFETs并联连接以增加整体系统电流能力，通常假设电流在并行器件之间同样分布或同样地共享。然而，PCB布局和特定MOSFET参数的若干特性可以影响该分布，并且只要参数与所述参数完全匹配或者布局不完全对称，就会对电流共享产生不平衡。

This article discusses the MOSFET parameters playing an essential role in the current sharing and addresses how design engineers can assess the required margin of output current capability for parallel MOSFET designs by evaluating the excess power losses incurred in the MOSFET that carries more current. The examples shown in this article refer to a half-bridge-based application, where MOSFETs are connected in parallel, as depicted in Figure 1.

图1.简化的半桥原理图，具有并联MOSFET

影响电流分布的MOSFET参数
When mismatched, specific MOSFET parameters influence how current is distributed among the paralleled devices in various ways. The value of RDS(on) has an effect during the conduction of the MOSFETs, while some other parameters (VGS(th), RG, CGS, CGD) influence current sharing during switching (see Table 1 [1] and read the related Application Note (see end of article) for full details).

表格1Parameters playing a significant role in current-sharing [1]

Quantifying the effect of mismatched MOSFET parameters on current and power dissipation imbalance
为了比较特定MOSFET特性或参数对整体系统性能的相关性，它们对性能的影响需要以对给定应用的方式进行量化。

电流共享不平衡意味着某些设备的导通高于平均电流。这导致高于平均电力耗散，并且因此，对于估计产生完善的电流共享和相等的组件功耗而导致的温度，这是较高的最大组分温度。

图2显示了具有最高耗散的MOSFET的功率损耗，在这种情况下，MOSFET是具有最低V的MOSFET_gs（th）。不同的线代表不同的v_gs（th）在我们的模拟中使用的不匹配[1]。ΔV_gs（th）= 1.6 V refers to the case when the V_gs（th）Q.₁比v低1.6 v_gs（th）Q._3.什么时候问₁and Q_3.are used in parallel (Figure 1).

图2.最热门MOSFET的平均功耗[1]

Assuming the thermal characteristics of the MOSFETs are the same, the MOSFET with the highest dissipation will always turn out to be the hottest one. The increased power dissipation and the excess component temperatures will therefore limit the maximum system performance. In this case, a certain amount of oversizing is necessary, which means that additional MOSFETs need to be added in parallel so that the system can meet the design requirements.

图3中绘制了具有最高耗散（即，最热的一个）的MOSFET的温度（即，最热的一个）。那些适用于具有T的散热温度的示例_HS.= 80°C. To make use of this graph, one must observe the output current at a particular device temperature. For example, if the system requirements allow for the maximum T_j（max）= 100°C，输出电流将受到相应的限制。

Figure 3. Temperature of the hottest MOSFET vs. output current (2 MOSFETs in parallel)

The resulting distribution of average power dissipation between the paralleled MOSFETs depends on the current distribution but is also affected by the nature of the output load current and the applied switching modulation.

For example, in电机驱动器应用，每个半桥的输出是正弦电流，所产生的功耗不同于DC应用。空间矢量调制（SVM）和正弦加权脉冲宽度调制（SPWM）是驱动正弦输出电流的典型调制方法[1]。可以利用各种调制技术对不同应用进行类似的分析。

Summary
本文展示了设计工程师需要考虑的示例，以便通过考虑“典型的”（即理想的）性能，而是由不可避免的组件参数变化引起的现实情况来尺寸。

Read the extensive应用笔记to gain more insights and browse examples as well as test results. Additionally, check out the available在线培训and please make sure to visit Infineon’s功率MOSFET网页。

References
[1]应用程序注意：AN_2009_PL18_2010_105641, “Paralleling power MOSFETs in high current applications,” Infineon Technologies, May 2021
[2]应用笔记：AN_1803_PL11_1804_092613，“在高电流LV驱动应用中的并行MOSFET，”Infineon Technologies，2018年4月。
[3] Forsythe B.詹姆斯，“功率MOSFET的平行于较高功率输出”，文章，国际整流器，2006年3月

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